1. Field of the Invention
The present invention generally relates to a gas laser angular rate sensor, also known and referred to in the art as a Ring-Laser Gyro (RLG), and more particularly to an improved system for monitoring laser beam intensity.
2. Description of the Prior Art
An integral part of a ring-laser gyro is the laser beam source or generator. One type of laser beam generator comprises electrodes and a discharge cavity in combination with a plurality of mirrors which define a closed path. The path is usually triangular but other paths such as rectangular can be used.
Present day ring-laser gyros employ a gas discharge cavity filled with a gas which is excited by an electric current passing between the electrodes ionizing the gas and creating a plasma. As is well understood by those skilled in the art, the ionized gas produces a population inversion which results in the emission of photons, and in the case of He-Ne, a visible light is generated which is indicative of the plasma. If the gas discharge cavity is properly positioned with respect to the plurality of mirrors, the excited gas will result in two counterpropagating laser beams traveling in opposite directions along an optical, closed-loop path defined by the mirrors.
In some embodiments of ring laser gyros, a unitary body provides the gas discharge cavity including the optical closed-loop path. Such a system is shown in U.S. Pat. No. 3,390,606 by Podgorski, which is assigned to the same assignee as the present invention. There an optical cavity is formed in a unitary block. A selected lasing gas is used to fill the optical cavity. Mirrors are positioned around the optical cavity at appropriate locations such that counterpropagating beams are reflected so as to travel in opposite directions along the optical cavity. A gas discharge is created in the gas filled optical cavity by means of an electrical current flowing in the gas between at least one anode and at least one cathode which are both in communication with the gas filled optical cavity.
It should be noted that prior art ring-laser gyro systems often have a pair of anodes and a single cathode which produce two electrical currents flowing in opposite directions. Each of the electrical discharge currents create plasma in the gas. Each current is established by an applied electrical potential, of sufficient magnitude, between one cathode and one anode.
In prior art RLGs, a photodetector optically coupled to one of the two counter-rotating laser beams produces a d.c. output signal whose magnitude is a function of the beam intensity. While satisfactory for some applications, in other applications this beam monitoring system is not satisfactory because of a varying d.c. bias in the sensor output caused by residual light generated in the gas discharge referred to in the art as "glow".
In prior art RLGs, energy from the two counter-rotating laser beams are optically folded and superimposed onto an optical detector. The two beams interfere with each other to form a moire fringe pattern. Differences in frequency of the two laser beams, caused by a rotation of the gyro, result in an alternating intensity of the light energy on the optical detector and corresponding a.c. output. This a.c. output corresponds to the angular motion of the gyro and forms the gyro output signal.